The present invention relates to a laminate layered photosensitive member having at least a charge generating layer and charge transporting layer provided on an electrically conductive substrate, and more specifically relates to a laminate photosensitive member having special characteristics employed in the formation of said charge transporting layer.
Function-separated type laminate photosenstive members have found wide acceptance in recent years because they possess certain advantages in that they can usually be manufactured by coating processes which are readily adaptable to production at low production cost, and the wavelength range of photosensitivity can be freely controlled by selecting suitable charge generating materials for use therein.
Irregular thickness of the layers in the photosenstive member, creeling defect of the photosensitive surface, and deterioration due to ozone and moisture on the image can lead to nonuniform image density in such laminate photosensitive members, so that when several hundred copies are continuously made the image density varies and the image fades.
The aforesaid problem becomes quite severe in the case of laser printers and the like which require high image reliability and stability during repeated printings, so that there is also a demand for photosensitive members suitable for use in said laser printers.
Major causes of the aforesaid problems are found in the coating of the photosensitive layer and particularly in the charge transporting layer thereof and are due, for example, to coating impurities and damage, friction and deterioration induced mechanically or by external physical force during resistance printing. It is also known that the properties of the binder resin used during formation of the charge transporting layer exert a great influence on the occurrence of the aforesaid problems.
Accordingly, there has been heretofore a wide range of research performed on the binder resins used in the charge transporting layer formation, and polycarbonate resins have found wide application as binder resins since it is well known that they are excellent in the whole range of electrophotographic properties such as wear resistance and photosensitivity.
When a single polycarbonate resin is used as a binder resin, however, the molecular weight varies by the batch. On one hand, polycarbonate resins with high molecular weights have excellent wear resistance, but it is difficult to remove toner which adheres to their surface, and image noise commonly occurs by filming development. On the other hand, polycarbonate resins with low molecular weights readily allow toner removal, but they possess poor wear resistance, and are susceptible to deterioration through exposure to ozone.
Japanese Patent Application No. 62-160458 recently proposed for use as a binder resin the use of a blend of two polycarbonate resins in suitable proportions, one resin having a mean molecular weight of 1.5.times.10.sup.4 or less, and another having a mean molecular weight of 4.5.times.10.sup.4 or greater.
However, when polycarbonate resin having a mean molecular weight of 1.5.times.10.sup.4 or less is added, the material becomes partially more susceptible to wear and deterioration induced by ozone and leads to partial image noise. When polycarbonate resin having a mean molecular weight of 4.5.times.10.sup.4 or greater is added, the coating fluid sets readily with wide variation in viscosity which shortens pot-life. Thus, when two polycarbonate resins having widely different molecular weights are combined, it is difficult to control viscosity of the coating fluid, and coating layer uniformity suffers.